The invention relates to the distribution of signals and, more particularly, to the survivable broadcast distribution of signals.
A variety of signal distribution applications may require the broadcast distribution of signals. For example, television, motion picture, radio, or other communications signals that are typically broadcast via the airwaves may also be distributed through a telecommunications network. To avoid major service disruptions, a telecommunications system that broadcasts signals should provide for some form of network survivability, to accommodate the failure of an element or link within the network. A telecommunications system having a loopback ring architecture, that is, a system in which signals traveling in one direction around a ring are rerouted in the opposite direction for delivery in the event of a failure, may provide some degree of survivability. However, such a self-healing system could inadvertently distribute signals to locations that are not supposed to receive those signals. In some cases, such rogue signals could be squelched to prevent their improper distribution. Such squelching is discussed, for example, in U.S. Pat. No. 5,442,620 which issued to the same inventor and is assigned to the same assignee as the present invention and which is hereby incorporated by reference. Although the squelching employed by such a system operates well under some circumstances, it may prove inadequate in a system that includes a plurality of signal sources.
Although conventional squelching approaches may be inadequate for multi-sourced, broadcast systems, neither is the forsaking of squelching a solution. Without squelching in such a network, an xe2x80x9cinfinite loopxe2x80x9d may be established in the ring transmission system, whereby network equipment could constantly alter pointer values or vacillate between an indication of the presence or loss of a pointer. Additionally, in order to be compatible with existing systems, which employ squelch tables it would be advantageous for a new communications system to employ a compatible squelching mechanism.
There is, therefore, a need for a broadcast distribution telecommunications system that provides survivability, prevents the improper distribution of signals, and circumvents the creation of infinite loops in the event of a plurality of source failures.
In accordance with the principles of the present invention, a network communications node for use in a broadcast ring communications system includes a controller that is responsive to indications that a relative first source node, that is, the source node farthest upstream, and a relative last source node, that is, the source node farthest downstream, in the communications system have failed by blocking communications through the node. In an illustrative embodiment identifiers of the relative first source node and relative last source node are stored within the network communications node in the form of entries in a squelch table. However, unlike conventional squelch table entries, which indicate the first entry point and last exit point in a circuit, the squelch table entries within the new communications node identify the relative first source node and last source node. The node blocks communications signals through itself only when both its relative first and relative last source nodes have failed.
The new node is particularly suited for a broadcast line-switched ring communications system that includes a plurality of sources. In accordance with the principles of the invention, each source within such a system would have associated with it a node, referred to as a source node, that is configured to operate as a drop-and-continue node. Each source within such a system accepts signals from a source and broadcasts the signals to drop-and-continue nodes in a given direction around the ring. Additionally, each source node accepts signals from another source node through a connection to a drop and continue node, with the signal routed around the ring from one source node to another in the same direction, that is, clockwise or counterclockwise. Each node within the ring has a relative first source node and a relative last source node associated with it. A node""s relative first source node is the source node farthest upstream from the node and the node""s relative last source node is the source node farthest downstream from the node. In a ring communications system in accordance with the principles of the present invention the node adjacent a failed node is a switching node. That is, the node performs a loopback switch in order to preserve as much of the ring as possible. However, if a switching node detects the failure of both its relative first source node and its last source node, the node blocks communications, rather than performing the ring loopback.
The new node, and bidirectional line-switched ring communications systems that may employ such a node, are particularly suited for use in the distribution of high-bandwidth signals, such as television, near video on demand, and other such communications. For example, a broadcast television distribution system could employ two of the new nodes as source nodes, each of which receives television signals for broadcast from a television headend. One or more of the new nodes may be positioned in a branch of the ring between the first and second source nodes, with the television signals being transmitted from the first headend through the first node, which is configured as a drop-and-continue node. From the first node, the television signals are sent, in a given direction, to the node(s) lying between the two source nodes. Each of the nodes lying between the source nodes acts as a drop-and-continue node, distributing the television signals locally, and passing them along to the next node in the loop. At the opposite end of the branch, another node, the other source node, accepts the television signals that have made their way through intervening drop-and-continue node(s). This source node may drop, or distribute, the received television signals locally. Additionally, this source node at the opposite end of the branch accepts television signals from a second television headend and distributes the signals in the same direction, (i.e., clockwise or counterclockwise) to additional node(s) in a second branch. The node(s) in the second branch operate in the same manner as those in the first branch, (i.e., drop-and-continue) and the signal from the second source node makes its way to the first source node. Should a node fail, the adjacent upstream node, referred to as the switching node, performs a loopback so that television signals may be distributed on the other side of the failed node. Each operational ring node continues to operate in a drop-and-continue mode unless both source nodes fail, in which case the switching node blocks signal transmissions. This blocking may be effected in a synchronous optical network (SONET) implementation, for example, by sending alarm indication signals (AIS) along the protection the system""s protection channel.
A broadcast distribution ring in accordance with the principles of the present invention may include more than two sources. In such a multi-source network each node may operate as a drop-and-continue node with substantially the same loopback operation as just described, except that the switching node looks to its relative first and last source nodes to determine whether to block signal transmissions. The relative first and last source nodes are, respectively, the closest downstream source node and the closest upstream source node. The failure of a node may indicate that the path to the node has failed, or that the performance of the node itself has degraded to an unacceptable level. Such a failure could be detected by the expiration of a timer or by the failure of a xe2x80x9cHelloxe2x80x9d signal or other conventional failure detection techniques. The multi-sourced broadcast ring architecture just described provides survivable service through it""s redundant sources. That is, if one source fails, one or more surviving sources may be used, via loopbacks, to supply signals to the node(s) which had previously obtained signals from the failed source, and, the nodes block transmissions to prevent the unwanted delivery of signals. Furthermore, the architecture could be used to supply varying levels of service to the nodes within the network. For example, one or more sources may supply a more expansive list of offerings than the other sources. In that case, the survivability for the premium source""s offerings may be limited to a subset offered by a source which provides redundancy for the premium source.
Each node within such a system includes a ring map, which depicts the interconnection of nodes within the ring. The ring map also includes a squelch table that contains the addresses of the relative first source node (the xe2x80x9cAxe2x80x9d address) and relative last source node (the xe2x80x9cZxe2x80x9d address) for each circuit that is inserted, passes through, or is dropped at the node. Each switching node looks up the A and Z addresses and squelches all circuits (e.g., inserts AIS in the circuits"" time-slots) that pass through the node, should the A and Z nodes fail.